In general, an automatic transmission refers to a transmission which automatically changes speed according to a traveling speed, and can perform optimal torque conversion without a clutch. The automatic transmission may be implemented with a combination of a torque converter and a planetary gear.
Currently, 6-8 speed automatic transmissions have been frequently adopted and applied instead of conventional 3-4 speed automatic transmissions, in order to improve fuel efficiency. Furthermore, the automobile industry has conducted much research to develop 9-10 speed automatic transmissions. By around 2013, the 9-10 speed automatic transmissions are expected to be commonly used.
However, as the number of speed stages is increased, the number of parts and the weight of the multi-speed automatic transmission are also inevitably increased. Thus, the automatic transmission has difficulties in improving fuel efficiency. That is, when the weight of the automatic transmission is increased, the effect of the improvement in fuel efficiency may be offset even though the multi-speed structure is applied.